Various types of downhole packers have been devised over the past century, including inflatable packers, compression set packers, and swab cup packers. One form of a compression packer with a central rubber section and upper and lower rubber sections having a higher durometer than the central section is disclosed in U.S. Pat. No. 4,161,319. Several embodiments of compression set packers include petal shaped end ring elements which bend and thus radially expand in response to high compressive forces, so that the effective diameter of the end ring when the packer is run in the well is less than the effective diameter of the end ring when the packer is set. These type of expanding end rings are complex, and require a significant axial force to deflect these petal shaped elements while setting the compression set packer. Various types of compression set packers, for example, require an axial setting force in excess of 50,000 pounds to reliably set the packer.
In more recent years, swellable packers have been commercialized which expand in response to downhole fluids, and thus do not require a setting mechanism or a setting operation. Suitable examples of swellable packers include U.S. Pat. Nos. 3,502,149, 4,137,970, 4,633,950. More recent patents and publications are U.S. Pat. No. 5,195,583, U.S. Publication 2004/0020062A1, WO 02120941A1, and EP1315883B1.
A swellable elastomer typically has a low modulus of rigidity and a low molecular weight, and accordingly will flow axially if a high pressure differential is applied to one end of the swelled elastomer. The swellable elastomeric element of a packer may be partially prevented from extruding axially during radial expansion by a rigid end ring secured to the packer mandrel and resisting axial extrusion of the elastomer. In some applications, the rigid end ring and the swellable element each have a diameter that is slightly less than the diameter of the well bore. Since the radial space between the O.D. of the swellable element and the borehole wall or between the O.D. of the swellable element and the I.D. of a larger concentric tubular may be about 3/16 inch or more, a reasonable differential pressure applied to the swelled element will cause flow or extrusion of the element into this radial space outward of the end ring, eventually negating the pressure seal.
The disadvantages of the prior art are overcome by the present invention, and an improved swellable downhole packer is hereinafter disclosed which maintains high reliability in response high downhole temperatures and pressures.